Background: Transmission of the SARS-CoV-2 virus in human milk (HM) is unknown with only 11 poorly-described studies, reporting detectable virus in just 1 of the total 36 breastmilk samples. Further, it is possible that maternal SARS-CoV-2 infection during pregnancy and lactation confers some specific protection through antibodies. Only one study reports the presence of IgG to SARS-CoV-2 in HM. Alarming examples of antibody-dependent enhancement of other viral infections illustrate how critical it is to understand both the profile and activity of these antibodies in HM. It is critical then to understand the transmission risks and protective factors between mother and child through breastfeeding. Design: We propose a longitudinal cohort study of COVID-19+ mothers to determine risks of transmission (viral exposure of infants through breastfeeding by presence in HM and on areolar skin) and protective factors (SARS-CoV-2-specific antibody response in HM, profile and neutralization capacity) of breastfeeding. Longitudinal samples will allow us to assess temporal changes over disease course. Methods: We will enroll 50 COVID+ mothers in the first 3 months of lactation, 25 symptomatic and 25 asymptomatic. We will concurrently collect samples from two locations: The University of Rochester School of Medicine and Dentistry and New York University, NY. These sites will provide robust and staggered access to patients. We will recruit through local hospitals and social media platforms. Mothers will provide informed eConsent. IRB approval has been received. All samples will be self-collected in the hospital (if admitted) or in the home. There will be no physical contact between study staff and participants. Mothers will express and collect whole breastmilk on Days 0 (enrollment), 3, 10, 19, 28, and 90, breast skin swabs on Days 0 and 3 and whole blood by fingerstick on days 0 and 90. They will also be instructed to provide a frozen HM sample from Day -7, if available. Analysis: RNA will be extracted from HM and breast skin swabs in the Jarvinen-Seppo lab. SARS- CoV-2 genomic RNA will be quantitated using RT-qPCR three amplicon probe-based system as recommended by CDC. We will compare presence and changes in SARS-CoV-2 in HM via repeated measures analysis. We will assess SARS-CoV-2 and other coronavirus antibodies in maternal finger prick and HM to S and N proteins of SARS1, SARS2, HKU1, 229E, NL63 and OC43 by Luminex on the mPLEX-CoV system developed and validated by the Zand lab. Repeated measures analysis will be used to compare the changes in concentration of HM anti-SARS-CoV-2 antibodies over the time-course of disease progression and also between women with mild to severe symptoms. Impact: These results are critical to understand the risks and benefits of breastfeeding in the context of COVID-19 infection, and are necessary to make evidence-based policies, and to care for these families.